"I arrived to the booth this morning with my wife to do a little restocking. It was about 8:10 or so this morning. When we opened up the booth, she noticed that some pieces were mising off of the walls. Like, 'Oh my gosh, what happened?' And we kind of looked around, and somebody took them," Jackson said.

Lex Covato had a sinking feeling when she opened her tent.

"The walls looked really bare," Covato said.

Someone had stolen $1,200 worth of her artwork, including a 16-by-20 foot painting on wood of Andy Warhol.

"It struck me as odd. I walked in and I wasn't expecting it, and I've done the show nine, 10 years. I love the show. I never thought anybody would steal anything," Covato said.

It's unclear when the thief or thieves got away with the artwork. The Arts Festival has 24-hour security and many of the pieces stolen were large.

"I'm kind of perplexed how you could get something that large out of the park here. It's not something you could hide underneath your jacket or shirt," Jackson said.

The Pittsburgh Cultural Trust issued the following statement about the thefts:

"These thefts are an unfortunate occurrence that is highly out of the ordinary for the Dollar Bank Three Rivers Arts Festival. The Pittsburgh Cultural Trust always implements security 24 hours a day during the Festival. Since learning of this activity, we have alerted our on-site staff, volunteers, and security partners in the city to be on special lookout for these acts and have dedicated further resources to security in the Artist Market area. If anyone sees something suspicious in the area, we encourage them to alert our public safety team, which can be reached in the public safety tent at the entrance of Point State Park or by calling 911."